Improvement of the selectivity, distortion, and signal-to-noise characteristics is a major goal of most communications receivers. Accordingly, FM detectors are now widely available which employ well-known techniques for achieving extremely good noise rejection and low distortion levels. Such extremely low distortion levels have not, however, been achieved in a very narrow band intermediate frequency ("IF") system. Prior art systems having the highest level of available performance relied on standard but extremely expensive techniques.
Two representative examples of prominent prior art designs are the Model 10B receiver manufactured by the Marantz Company and the Model 1 receiver manufactured by the Sequerra Company. Both of these prior art receivers used 18-pole LC coupled filters in the IF stages. These filters used pot core technology and were exceedingly expensive. Furthermore, although these receivers exhibited exceptionally good mid-band performance, they still suffered from the problem of high- frequency distortion. The Marantz and Sequerra systems achieved approximately 35-40 dB of adjacent channel selectivity and did not require any wide/narrow switching of the IF bandwidth. This performance was, however, achieved only at the great expense of pot core technology, which was typically too expensive for use in consumer products. The expense was increased further because the alignment procedure required for these types of IF circuits is both tedious and time consuming. A long-standing goal has therefore been to achieve equal or better performance more inexpensively.
One response to the need for a less expensive yet equally selective system has been the development of low cost ceramic filters. Such ceramic filters have in fact become standard in IF filter designs. Linearity of group delay is necessary to achieve low distortion, especially at high audio frequencies of approximately 6 kHz to 15 kHz. In conventional ceramic devices, linearity of group delay is generally achieved at the expense of bandwidth so that the bandwidth is too wide, and the slopes of the flanks of the filters are not steep enough to achieve good selectivity. In order to increase the slope of the flanks of the filters, many filters must be used in series; however, doing so causes a progressive increase of the non-linear group delay, which in turn leads once again to the above-mentioned increase in distortion at high audio frequencies. Additionally, such ceramic filters are not adjustable.
In view of the drawbacks of ceramic filters, some designers have reverted to using adjustable, equalizing LC filter sections in an attempt to offset or reverse the buildup of group delay distortion. A reversion to using LC filter sections, however, also means a reversion to the above-mentioned drawbacks of such filter sections. Furthermore, these efforts have generally not improved the spurious response rejection of the filter, since the stop band performance of ceramic filters is typically quite poor.
Another well-known technique for improving the performance of FM detectors employs negative feedback, from the output of the detector back to one or more local oscillators. U.S. Pat. No. 2,075,503 (Chaffee) describes a device which is representative for this class of detectors. All such detectors using negative feedback employ the concept of bandwidth compression or deviation modification ("DM"), but none of them address the most important aspect of the technique: the reapportionment of the spacing between the adjacent and alternate carriers next to the desired carrier. These techniques improve the distortion performance by reducing the deviation, but they still require the use of expensive filters in the IF stages. Furthermore, many of these negative feedback systems fail to alleviate the problem of image interference.
All prior art negative feedback systems use feedback from the FM detector to drive some form of reactance modulator, local oscillator, or voltage controlled oscillator ("VCO") in such a direction that the net carrier deviation through a mixer is reduced. This is, however, not true bandwidth reduction since the effective detected bandwidth remains the same. Because of their typical use of superheterodyne down conversion, these negative feedback systems exhibit poor if any image rejection. No superheterodyne technique can function properly without pre-filtering to remove the image frequency. However, with successive down conversions, the problem of image frequencies becomes progressively worse, since the frequency bands and VCO frequencies become closer, thus necessitating even more filtering; indeed, the increasingly stringent filtering requirements caused by the proximity of the carrier and VCO frequencies set practical limits on the achievable performance of these systems. Likewise, the greater the negative feedback which is applied from the detector back to the VCO, the more the deviation is reduced. Eventually, as in all negative feedback systems, the system gain is reduced to near unity, in which case the detector output approaches zero and renders the detector useless When dealing with the standard FM IF frequency of 10.7 MHz, a mathematically practical upper limit is reached for a deviation reduction factor of approximately five. This upper limit is also recommended by Turner, et al. in U.S. Pat. No. 3,053,981.
Further drawbacks of prior art negative feedback systems are illustrated by the following example:
Assume a standard FM IF frequency of 10.7 MHz with a deviation of .+-..DELTA.75 kHz. Assume further that a deviation reduction factor of five is used. It can be shown mathematically that the frequency translation of the superheterodyne mixer should also be of the same magnitude. For example, if the deviation is to be reduced by a factor of five from the original .+-..DELTA.75 kHz to .+-..DELTA.15 kHz, the frequency of the local oscillator should be 12.84 MHz, thus yielding a mixer output difference frequency of 2.14 MHz=10.7 MHz / 5. The new second IF frequency then becomes 2.14 MHz with a deviation of .+-..DELTA.15 kHz.
It is important to note that this down mixing must be preceded by some kind of filtering, either of the incoming radio frequency ("RF") or, preferably, of the incoming IF signal of 10.7 MHz. The reason for this is that the new image above the VCO will be at 14.98 MHz which will also produce an output from the mixer at the desired 2.14 MHz. Assuming the standard channel spacing of 200 kHz, the two carriers at 14.9 MHz and 15.1 MHz would thus be close enough to cause total interference and must be removed by pre-filtering. Plainly, if the reduction factor were increased from five to, e.g., seven, nine, etc., the problem of signal images would become significantly worse and increase the difficulty of filtering even further. Eventually, it would not be practically possible to create a sufficiently selective filter.
Accordingly, the object of the present invention is to provide an FM detector whose distortion and signal-to-noise characteristics are greatly improved compared to the prior art. The FM detector according to the invention achieves the following primary goals:
1. Significant reduction of co-channel interference through removal of interfering adjacent and alternative side bands.
2. Elimination of the need for expensive LC filters while greatly reducing distortion through the use of existing inexpensive ceramic filters, which require no tuning.
3. Avoidance of the necessity for extremely high order filters having steep slopes, without worsening the capture ratio of the detector.
4. Enabling isolation and/or measurement of multi-path interference.
5. Elimination of the problem of "birdie" beats, which typically occur in the stereo multiplex decoding sections of prior art FM receivers.
6. Elimination of the need for IF band switching and associated switching hardware.